Control of a well



United States Patent 1111 3,550,696

[72] Inventor John W. Kenneday [56] References Cited n6us:6n,.Tex.UNITED STATES PATENTS [2H pp 349,238 2,082,329 6/1937 Foran et al175/205 1 Filed July 25,1969 3,213,939 10/1965 Records 166/75 1 Patsmed3,268,017 8/1966 Yarbrough 175/25 1 Asslsnee E859 Product Research m3,338,319 8/1967 Griffin 175/25 Continuation of application Ser. No.625,767 Man 24, 1967 abandoned Primary Exammer.lames A. LeppinkContinuation-impart of application Ser. No. Schneider 517,036, Dec. 28,1965, now abandoned.

ABSTRACT: Well control method and apparatus for auto- [54] g Y F'matically maintaining an essentially constant bottomhole presm sure in awell borehole during circulation of drilling fluid from [52] U5. Cl.175/25, a surface inlet down a drill string located in the borehole andI 166/75 up the annulus between the drill string and the borehole wall[51] Int. Cl. ..F21b 33/03, to a surface annulus discharge bycontrolling the rate of F2lb 7/00 drilling fluid discharge from theannulus discharge in response [50] Field of Search 166/5, .6, tovariations of drilling fluid pressure at the surface inlet from 75, 267,244; l75/25, 57, 65, 205 a preselected fluid pressure.

Pmmmaczslsm I 3550.696

SHEEI 1 .UF 3

INVENTOR.

i I JOHN w. KENNEDAY,

@18- BYM/dfig.

ATTORNEY.

PATENIfinniczslsm 3,550,696

SHEU 3 [IF 3 GPOWER FLUID DRIVE Mu0 FROM PIT CONST'ANT RATE CONTROLLERif I SWIVEL -|7 DRILL PIPE\ PRESSURE 1 SENSING 1 coum RoLLzR 1 INVENTOR.

JOHN W. KENNE DAY,

ATTORNEY.

CONTROL A WELL This application is a continuation of Ser. No. 625,767,filed Mars. 24, 1967, by John W. Kenneday, entitled, Control ofa Well,"which was a continuation-in-part application based on application SerialNo. 517,036, filed Dec. 28, 1965, by John W. Kenneday, entitled Controlof a Well," both now aban- ,technique of well control. This technique ismore easily explained by considering a well bore containing a drill pipeas an open ended U-tube The bottomhole pressure is common to bothcolumns and is reflected in the drill pipe and annulus pressures.Variations in either pressure are transmitted through the respectivehydrostatic columns and these variations are of equal magnitude and inthe same direction. Applying this principle to the dynamic process ofcirculating an extraneous fluid (gas, oil and/or salt water) from awell, drill pipe pressure can be controlled by regulating the annulusback pressure at the surface such that a constant bottomhole pressure ismaintained.

The present invention may be briefly described as a method forautomatically controlling a well during drilling operations inwhich thedrilling fluid is circulated through a hollow drill string and up theannulus between the drill string and the wall of the well. In accordancewith the present invention, when a sudden surge of pressure isencountered, the annulus between the drill string and the wall of thewell is sealed. After the drill bit encounters the higher pressureformation, pressure is imposed on the circulating drill fluid in anamount sufficient to overcome the pressure in the drill string. Flow ofdrilling fluid from the annulus is controlled responsive to the pressureof drilling fluid into the drill string.

In accordance with the present invention,'the drilling fluid iscirculated by pumping to maintain a substantially constant bottomholepressure. Pressure of the circulating drilling fluid controls the amountof drilling fluid returned from the annulus.

In summary, the preferred steps of I the well control procedure may be:

( l Detect a well kick.

(2) Shut-in the well while rotating the drill pipe.

(3) Measure drill pipe pressure and annulus pressure at the surface.

(4) Start circulating the well with annuluspressure greater than (e.g.,100 p.s.i. greater than) shut-in annulus pressure.

(5) Observe drill pipe pressure at circulation rate of step (6)Circulate the well while automatically maintaining the drill pipepressure essentially constant at the observed drill pipe pressure. 4

In the present invention in which a hollow drill string is employed, andmeans for introducing drilling fluid into the hollow drill stringincluding a pump means and means for discharging drilling fluid from theannulus between the wall of the well are provided, the apparatuscomprises first valve means in the discharge means for controlling thedischarge of the drilling fluid at a controlled pressure. Means areprovided for supplying operating fluid under regulated pressure to thefirst valve means and a pressure sensing control means which may be apressure-responsive pilot (second) valve means provided on theintroduction means for controlling the pressure of drilling fluid intothe hollow drili String by controlling discharge of drilling fluid friomthe annulus with the first valve means. The pressure sensing controlmeans is operatively connected to the first valve means which isresponsive to the con-' trol means; pressure of the drilling fluidintroduced into the drill string controls discharge of drilling fluidfrom the annu lus. Thus, in accordance with the present invention, thecontrol means is operatively connected to the first valve means which isresponsive to the pressure of drilling fluid circulated past the controlmeans into the drill string.-

In addition, means may be provided for interrupting transmission of thecontrol signal responsive to changes in drill pipe pressure apredetermined amount of time (the time required for the pressure wave orpulse resulting from opening and closing of the first valve means totravel down the annulus and up the drill pipe to the control means) topermit the control means to sense the pressure effects in the drill pipecaused by previous changes in the position of the first valve meansbefore making an additional change in the position of the first valvemeans. This lag" time may vary, for example, up to 20 seconds for welldepths up to 10,000 feet, respectively. Lag time is a function of columnlength and pressure transmissibility factors, principally fluid density.

The present invention is quite important and advantageous in that entryof formation fluids, such as gas, into the well is balanced and controlof the discharge of extraneous fluids safely and automatically throughthe annulus choke (first valve means) line is provided while maintaininga selected bottomhole pressure. Thus, in accordance with the presentinvention, automatic control of annulus returns by circulating pumppressure allows unskilled personnel to handle a sudden increase inpressure due to gas entry without running into the danger of stickingthe pipe and encountering lost returns. Also, by controlling bottomholepressure, further entry of formation fluids is avoided andformationfracturing is prevented by avoiding excessive pressure on thewell by permitting fast and efflcient well control and release ordischarge of the unwanted fluids. Additionally, by having the pumpoperation at a preselected (constant) rate (or rates) and by controllingoperations relative to the hydrostatic column in the well,

operations may be conducted automatically. The advantage of a constantrate is that it makes circulating friction loss constant.

The present invention will be further reference to the drawing in which:

FIG. 1 represents schematically one mode and embodiment of the presentinvention;

FIG. 2 illustrates schematically another embodiment of the invention;

FIG. 2A is an enlarged, partly sectional view of one of the componentsshown in FIG. 2;

FIG. 3 illustrates schematically still another modification of theinvention;

FIG. 3A is a detailed view of the pressure sensing controller apparatusillustrated in FIG. 3; and

FIG. 3B is a detailed view of another pressure control componentillustrated in FIG. 3.

Referring now to the drawing which represents a best mode described withl and embodiment and particularly in FIG. 1, numeral 11 designates theearths surface into which 192 well 196 has been drilled comprising aportion 13 in which a casing 14 has been cemented with cement 15 and anopenhole portion 16 in which the drill string 17 including a bit 18 isarranged. The drill string 17 is suspended in the well (by means notshown) from the drilling rig l9 and the drill string is rotated bysuitable rotary table 20 resting on a foundation 21. In the cellar 22 ofthe well, a blowout preventer 23 is provided for sealing off the casing14. Blowout preventer 23 is preferably of the type which allows piperotation. The'casing 14 is provided with one or more flow or choke lines24 in which a pressure-responsive valve 25 is provided for controllingthe discharge from the annulus A in the well 12. The flow line 24discharges into a mud pit 26 whence suction is taken through line 27into'pump 28. Line 29 discharges drilling fluid from pump 28 to a swivel30 and thence down through the drill string 17 and up the annulus A ashas been described.

Arranged on the conduit 29 is a pressure sensor control 32 which may bea pressure-responsive diaphragm or piston or a pressure-responsive pilotvalve. Power fluid, such as air or any other gas, at a selected pressureis introduced by line 34 against a diaphragm or piston inpressure-responsive valve 25 to allow the valve 25 to open and/or closein response to variations from a preselected drill pipe pressure.Pressure from sensor 32 is transmitted by line 35 against the diaphragmor piston or other pressure biasing means, such as a spring ofpressure-responsive valve 25, such that pressure-responsive valve 25 mayeither open or close (throttled) to allow more or less drilling fluid tobe discharged through line 24 into mud pit 26.

Pressure sensor 32 transmits pressure increases in drill pipe 29 above apreset selected pressure to the pressure-responsive means in valve 25,which is balanced by the power fluid. Decreases in drill pipe pressurecause the power fluid to close down automatically on valve 25. Firstvalve means 25 (motor valve) may be of the type shown and described onpage 1834 of the I962-63 Composite Catalog of Oil Field Equipment andServices type I25PP or I25PA (Fisher Governor Company).

Alternatively, a Bourdon type tube type regulator with reset (timedelay) and a motor valve could be used. The Wizard type controllerillustrated on page 1836 of the 1962-63 Composite Catalog of Oil FieldEquipment and Services" modified by pressure sensing and controlsignaling piping redirected to sense and signal as described laterherein with respect to the embodiment of FIG. 3 would be satisfactory.

A directory powered input pressure-responsive controller motor valve(combination unit) which would be suitable for use as the pressuresensing and valve elements is shown on page 2168 of the I96263 CompositeCatalog of Oil Fuel Equipment and Services.

Thus, in accordance with the present invention, assuming blowoutconditions to exist in the well (mud hydrostatic column is insufficientto overbalance formation pressure in, for example, a permeablegas-bearing zone), the blowout preventers are closed to seal the annulus(the pump may be shut down to halt circulation) and drill pipe pressureis read as by gauge 38 connected to line 29 to determine the requiredincrease in mud hydrostatic column which is the minimum necessary tobalance the bottom hole formation pressure. The well is then circulated,discharging from the annulus through the choke line 24 using pumppressure to overcome formation pressure. Pilot valve 32 andpressure-responsive throttle valve 25 control discharge of fluid fromthe annulus.

In practice, when a sudden increase in pressure is noted in the well,such as by influx of gas from a permeable formation, such as 37encountered by the drill bit 18, the blowout preventer 23 is closed toseal the annulus A. The influx of extraneous fluid, such as gas, into awell while drilling may be detected by 1) increase in mud pit levels;(2) increase in rate of mud returns; (3) unexpected sustained drillingbreak; (4) swab check with drill pipe; or (5) fillup and flowbackmeasurements during trips. Once fluid influx is detected. the pump isshut down and the well is shut in. The drill pipe preferably continuesto rotate. Drill pipe pressure. readings are made from gauge 38. Pump 28is operated at a selected essentially constant rate (or rates if pumprate is to be periodically changed) and controls the flow of drillingfluid through conduit 29 to drill string 17 by virtue of pressure onvalve 25 which would be opened or closed, depending on the variations influid pressure from pilot valve 32. This series of operations allows theadvantages ofthe present invention to be obtained.

In the modification of the invention illustrated in FIG. 2, a dampenerunit 72 is located in line 35. This unit may be used to interrupttransmission of pressure pulses from conduit 29 to an input pressurecontroller 71 which controls the signals to the pressure-responsivemeans 36 in throttle valve 25. Dampener 72 may be any well-known,commercially available apparatus suitably of the dash pot type providedwith an adjustable orifice to regulate the time required to return topressure transmitting position. The time delay means, although shown asa dampener positioned in line 35 for purposes of illustration, may bepart of the signal line 73, controller 71 or valve motor 36. Also shownin FIG. 2 are a manually operated valve 31 and a pressure gauge 33located in choke line 24 upstream of throttle valve 25. AIso,-a valvemay be provided in line:

35 as shown.

A suitable dampener is illustrated in FIG. 2A. A housing 75 containschambers 76 and 77 separated by a barrier containing an opening 79through which a rod 80 extends. Seals 81 are arranged in opening 79 toprevent fluid communication between chambers 76 and 77. A piston 82provided with a small equalizing orifice 83 is arrangedon one end of rod80 in chamber 76 between inlet line 35 and signal line 84. A compressionspring 85 biases piston 82 to the right as shown in FIG. 2A. Anotherpiston 86 is arranged in chamber 77 on the other end of rod 80. Piston86 is provided with a flapper valve 87 containing an orifice 88 whichopens with movement of piston 86 to the left and closes with movement ofpiston 86 to the right as shown in FIG. 2A. Conduit 84 contains afloating piston (or diaphragm) 90 which is used to avoid mixing offluids.

As shown in FIG. 2A, piston 82 is in balanced or static position inwhich position P =P When P becomes greater than P because of increasedpressure in conduit 29, piston 82 is pressured rapidly to the left tocover the opening of signal conduit 84 into chamber 76 which causesspring 85 to compress and flapper valve 87 to open as piston 86 is movedto the left. Then spring 85 slowly moves piston 82 to the right againstthe resistance of fluid passing through orifice 88 to return piston 82to its static position at which P equals P The time required for piston82 to return to its static position as it is delayed by passage of fluidthrough orifice 88 in valve 86 is the lag time. A decreased pressure P,will result in a similar operation, except lag time is measured as thepiston 82 initially moves to the right.

In operation, flow dampener 72 interrupts passage of pressure pulsestransmitted through line 35 for approximately the amount of timerequired for a pressure pulse to travel from throttle valve 25 downannulus A and up drill string 17 to line 35. Such delay or lag time isdesired in order to reduce the magnitude of the pressure variationswhich occur from actuation of throttle valve 25 in response to drillpipe pressure variations sensed by pilot valve 32. The "lag time permitsthe pressure signal from line 29 to actuate and control the position ofthrottle valve 25 at the time it has sensed the pressure in line 24resulting from a previous change in the position of valve 25.

Referring now to FIG. 3 in which identical parts are identified byidentical numerals as those in FIG. 1, in this mode and embodiment,means are provided for automatic setting of pump pressure control levelas a function of pump rate to compensate for changes in circulatingfluid friction. The embodiment of FIG. 3 provides for (I) automaticallymaintaining essentially constant drill pipe pressure by controllingannulus fluid discharge responsive to drill pipe pressure therebykeeping the bottomhole pressure essentially constant (embodiment ofFIGS. land 2) plus (2) means for automatically adjusting the drill pipepressure an amount equivalent to changes in circulating frictionresulting from changes in pump rate, thereby keeping bottomhole pressureessentially constant for different circulating rates. Automaticadjustment of the preselected drill pipe pressure to compensate forchanges in circulating friction can be achieved by various mechanical,electrical, hydraulic-mechanical, pneumaticmechanical or fluidic controldevices well known in the art which sense input circulating rate (pumpstroke counter, meter circulation rate, measure input power consumption)and issue a control signal. Control is, in turn, readily correlated toand calibrated for circulating friction by mechanical, electrical,hydraulic, pneumatic or fluidic devices.

One desirable manner of hydraulically-mechanically adjusting thepreselected drill pipe control pressure level is illustrated in FIG. 3.A circulating pump 28 takes suction through line 27 from mud pit 26 asdescribed with respect to FIG. 1. Pump 28 circulates drilling fluidthrough line 29 and through drill string 17 as described in FIG'. 1 andup annulus A and thence through line 24 which is controlled by athrottle valve 40. Throttle valve 40 is actuated through a diaphragm4land a biasing or spring means 42. The top side of diaphragm 41 has powerfluid working against the diaphragm. 41 which is introduced theretothrough line 43 while power fluid is introduced on the underside ofdiaphragm 41 aiding the spring 42 by line 44. This will be described inmore detail hereinafter. Line 45 branches from line 29 and-connects intoan input pressure sensing controller generally indicated by the numeral46 and shown in detail in FIG. 3A. Input pressure sensing controller maybe of conventional type. A description of a suitable controlleradaptable for use in the system of FIG. 3 may be found on page 1836 ofthe l962--63 "Composite Catalog of Oil Field and Pipeline Equipment." Asseen in FIG. 3A, the input pressure sensing controller comprises aBourdon tube 95 which is connected to line 45 and to a sliding valve 96by means of linkage 97. Sliding valve 96 is provided with upper andlower valve elements 98 and 99, respe ctively. Valve element 98 isarranged for movement within an inner movable cylindrical valve member100 which is connected to red 66 and movable thereby; Cylinder 100 isprovided with ports 101 and 102 which, depending upon the position ofvalve element 98, control flow of fluid, between l conduits 47a and 44.Similarly, valve element 99 is positioned within a cylindrical valvemember 103 provided with ports 104 and 105 which control, depending uponthe positionof valve element 99., the flow of fluid between conduits 47band 43. Cylinders 100 and 103 are connected together for unitarymovement.

Thus, a change'in the position of rod 66 causes the positions ofcylinders 100 and 103 to change and change in the position of rod 97moves valve elements 98 and 99 which in turn control flow of fluidsthrough conduits 44 and 47a and 43 and 47b. The source of power fluid 48may be suitably a gas bottle under pressure. The power fluid is suppliedthrough valves actuated by the Bourdon tube of controller 46 to lines 43and 44 through the slide valve which in turn is actuated by positioningrod 66 as will be described further.

Operatively connected by power means. 50 to pump 28 is a power fluidpumpSI whose pumping rate is proportional to the pumping rate of pump28. Pump 51 receives power fluid by way of line 52 from a source notshown to obtain a pressure output which is essentially proportional tothe change in power fluid volume output by discharging most of the powerfluid through constant rate controller 54 and the remainder throughchoke control valve 73. This power fluid may suitably be a hydraulicfluid, but other suitable. fluids may be used. The discharge from-pump51 is by way of line 53 to a constant rate controller 54 which suitablyis a constant rate controller such as described on page 2l68 of the1962-63 Composite Catalog of Oil Field and Pipeline Equipment," withpower fluid being discharged therefrom to a source by line 55. A branchline 56 leads from lines 53 and provides power fluid under a variablepressure, which pressure is varied proportionally to changes in powerfluid pump 51 volume output and hence proportionally to changes incirculating pump 28 volume output.

Thus, power tluid pump 5] pumps at a rate directly proportional to thepump rate of circulating pump 28. This proportional rate of power fluidoutput (proportional volume) is used to obtain a pressure put outputwhich is proportional to change of volume output by throttling powerfluid through choke control valve 73. Variable pressure upstream of thechoke, which is thus made proportional to change of circulating pump 28power fluid volume, is introduced on one side of actuating piston 63.Well control circulating pressure in line 29 is introduced onto theopposing piston area of actuating piston 63 through lines 59-.-60.Movement of piston 63 responding to changes in the difference ofpressure across choke 73 and relative to changes in the output volumesof pumps 51 and 28 are thereby converted to linear motion of connectinglinkage 66. Linear motion of linkage 66 readjusts the preselected drillpipe control pressure level of controller 46. The linear linkage drivewill shift the null" or control I pressure level an amount equal to thechange in circulating friction due to change in output of pump 28.Controller 46 will then control to the new control pressure level (ornew null" position) without influence from linkage 66 until operatingrate of pump 28 is again varied. g

Constant rate controller 54, differential pressure sensor 57, checkvalve 62 and discharge line 59 are devices to facilitate conversion ofproportional power fluid volume to a proportional change in power fluidpressure and subsequently to a proportional linear movement, to exhaustexcess power fluid, and to avoid backflow of well control fluid fromline 29 into the power fluid lines, controls and storage shouldequipment malfunction. Differential pressure sensor 57 shown in detailin FIG. 38 includes a spring-biased piston -connected to a valve member111 having a valve opening 112 by hollow rod 113 provided with a port114 which fluidly communicates lines 59 and one side of piston 110 tobalance pressure in lines 59 and 56. Q

Constant rate controller 54 receives fluid from pump 51 and line 53 andreturns a major portion of' the circulating power fluid to storage byline 55. Controller 54 will normally be set to bypass full discharge ofpump 51 at the minimum desired operating rate of pump 28 and. willthereby minimize discharge of power fluid through lines56,-6l, 59 andinto the well circulating fluid input line 29. In addition, thisoptional :mode of operation of rate controller 54 increases power fluidthrough input line 56 from zero for minimum operating level of pump 28directly relative to the differential rate of operation of pump 28. Nullpreadjustment of throttling valve position for some operating level forpump 28 adjusts back pressure imposed on the bottom area of piston 63 toopposed pressure from circulating pump pressure in lines 29, 59, 60 andimposed on the top area of piston 63 at the preselected inputcirculating control level. For each operating rate of pump 28, excesspower fluid from pump 51 is exhausted at a constant rate and constantpressure by valve 73 into line 59 and into the circulating system line29.

Under Null" condition, the override control system to adjust circulatingpressure control pressure level is balanced out and exerts no control.At this preset drill pipe pressure level or null position, the variableinput rate automatic well control system functions exactly as if it werea constant input rate system. Thus, it senses input pressure throughline 45 with Bourdon tube type controller 46 supplying control signalsto annulus discharge vale valve 42 to control annulus fluid dischargeflow and pressure and thereby adjust pressure in line 29 to the desiredcontrol pressure level.-

At some other circulating rate (above preestablished minimum) for pump28, excess power fluid from pump 51 and line'53, which is not bypassedby controller 54, is forced through line 56, pressure sensor 57 andcheck valve 62. This excess volume in throttling through choke valve 73builds back pressure in line 61 in proportion to the the throttling flowrate. This increased pressu reupstream of choke valve 73 is exerted 0nthe bottom piston area of piston 63 and the pressure sensing pilot 57.Excess power fluid from pump 51, not bypassed through controller 54, isthrottled through choke valve 73 at a changed rate and pressure anddischarged into circulating system line 29. Check valve 62 prohibitsback flow of well circulating fluid into the power fluid system. Pilotsensor 57 senses pressure upstream of check valve 62 in line 61 andimmediately downstream of throttling point in choke valve 73 to controlthrottle valve- 73. Differential pressure control adjustment correctsthe range of operating back pressure on one end of piston 63 to opposethe range of circulating pressure-from line 29 imposed on the other endof piston 63 for correct positioning of piston 63 to exactly the desiredadjustment in control pressure level (null position).

Thus, the variable pressure of power fluid also acts on sensor valve 57and is opposed by circulating pump 28 pressure.

These pressures act to adjust valve 73 and to power fluid to piston 63.Branch line 59 leads to sensor valve 57 and connects by line 60 toactuator 58. Line 61 connects actuator 58 with valve 57 and alsoconnects with line 59, which is provided with a one-way check valve 62.The actuator 58 is provided with a piston 63 which is biased downwardlyby spring 64 and biased upwardly by spring 65. Rod 66 connects to thepiston 63 and transmits vertical motion (as shown in the drawing) to theinput pressure sensing controller.

In the mode and embodiment of FIG. 3, the pump 28 pumping drilling fluidto the drill string 17 causes circulation from the annulus A throughline 24 back to the mud pit 26.When formation fluids from formation 37begin to flow to the open hole 16 and to the annulus A, the blowoutpreventers 23 are closed and the valve 40 is throttled. As the pressurein line 29 is transmitted to actuating means 58 and to the inputpressure sensing controller 46, the input pressure sensing controller 46and the actuator 58 cause throttling of the valve 40, the actuating rod66 allowing power fluid pressure to be asserted above or below thediaphragm 41 to close or open the valve 40, as the case may be. Thiscontrol means 46 thus is operatively connected to the control throttlevalve 40 which also is a control means and compensation is made tomaintain a selected overburden pressure and to compensate for drillingfluid rate. By sensing the pressure changes due to changes in the fluidin the open hole 16 and annulus A, it is possible to maintain thepressure at a selected point to prevent the sudden surges. Thus, inaccordance with the mode and embodiment of FIG. 3, compensation is madefor changes in drilling fluid rate due to the changes in the friction ofthe drilling fluid as it flows through the pipe.

The hydraulic-mechanical system described herein and shown in thedrawings is not the only manner and means contemplated for achievingautomatic well control in accordance with this invention. Various typesof mechanical, electrical, pneumatic or fluidic systems and combinationsthereof may be substituted for the hydraulic-mechanical systemillustrated as will be understood by those skilled in this art. Forexample, an electric-pneumatic or electric-hydraulic system whichcomprises an electric control panel, pressure transducers (to convertpressure measurements to electrical signals) and sole noids (to convertelectrical signals to pneumatic or hydraulic signals) may be used tocontrol automatically operation of a pneumatically or hydraulicallyoperated adjustable choke in the annulus discharge line in response tofluid pressure changes in the well inlet line. In such a system, a lag"time correction adjustment can be made directly on the control panel.

The nature and objects of the present invention having been fullydescribed and illustrated and the best mode and embodiment contemplatedset forth, what l wish to claim as new and useful and secure by LettersPatent is:

lclaim: 1. A method for controlling a well during drilling operations inwhich the drilling fluid is circulated from a pump to a hollow drillstring, through the drill string and up the annulus between the drillstring and the wall of the well to establish a column ofdrilling fluidwhich comprises:

sealing the annulus when pressure in the well increases to a pointsufficiently great to overcome the hydrostatic pressure ofthe columnofdrilling fluid in the well;

automatically adjusting the pressure of the circulating drilling fluidto compensate for increased well pressure; and

circulating drilling fluid at said adjusted pressure from the pump intothe drill string at a controlled, substantially constant rate whiledirecting all of the drilling fluid flowing from the pump into the drillstring and while simultaneously controlling the discharge of drillingfluid from the annulus in direct response to variations in the pressureof the drilling fluid in the drill string, thereby controlling the flowon the annulus whereby the pressure on the drilling fluid introducedinto the well is varied and said flow is controlled.

2. A method in accordance with claim 1 in which the 4. In a method forcontrolling a well in which drilling fluid is pumped from a pump to asurface inlet of a drill string, down the drill string and up theannulus between the drill string and the borehole wall to a surfaceannulus discharge by maintaining constant bottomhole pressure theimprovement compris ing; automatically adjusting the annulus circulatingfluid discharge rate higher and lower in response to increases anddecreases, respectively, in the inlet circulating pressure whilemaintaining input flow rate of drilling fluid into the drill stringsubstantially constant, and while directing all of the drilling fluidflowing from the pump into the surface inlet.

5. A method as recited in claim 4 including the step of reducing themagnitude of the pressure variations resulting from variations in therate of fluid discharge from said annulus in response to variations ininlet circulating pressure.

6. A method for controlling awell to compensate for changes inbottomhole pressure when drilling wells in accordance with rotarydrilling techniques in which drilling fluid is circulated from a pump toa surface inlet of a drill string, down the drill string and up theannulus between the drill string and the borehole wall comprising thesteps of; automatically and simultaneously maintaining essentiallyconstant drill string pressure by directly controlling annulus fluiddischarge responsive to drill string pressure while maintaining inputflow rate of drilling fluid essentially constant while'directing all ofthe drilling fluid flowing from the pump into the surface inlet, therebykeeping bottomhole pressure essentially constant.

7. In a method for controlling a well in which drill fluid is pumpedfrom a surface inlet down the drill string and up the annulus betweenthe drill string and the borehole wall to a surface annulus discharge bymaintaining constant pressure the improvement comprising:

automatically adjusting the annulus circulating fluid discharge ratehigher and lower in response to increases and decreases, respectively,in the inlet circulating pressure for a selected pump rate;automatically adjusting drill string pressure an amount equivalent tochanges in circulating friction resulting from changes in said selectedpump rate; and

automatically adjusting the annulus circulating fluid discharge ratehigher and lower in response to increases and decreases, respectively,in the inlet circulating pressure for said changed pump rate, therebykeeping bottomhole pressure essentially constant for different pumprates.

8. A method as recited in claim 7 including the step of reducing themagnitude of the pressure variations resulting from variations in therate of fluid discharge from said annulus in response to variations ininlet circulating pressure.

9. A method for controlling a well to compensate for changes inbottomhole well pressure when drilling wells in accordance with rotarydrilling techniques in which drilling fluid is circulated down the drillstring and up the annulus between the drill string and the borehole wallcomprising the steps of:

automatically and simultaneously maintaining substantially constantdrill string pressure for a selected fluid circulating rate by directlycontrolling annulus fluid discharge responsive to drill string pressure;automatically adjusting drill string pressure an amount equivalent tochanges in circulating friction resulting from changes in pump rate; and

automatically and simultaneously maintaining substantially constantdrill string pressure for said changed rate by directly controllingannulus fluid discharge responsive to drill string pressure, therebykeeping bottomhole essentially constant for different circulating rates.

l including the step of 10. A method as recited in claim 9 including thestep of reducing the magnitude of the pressure variations resulting fromvariations in the rate of fluid discharge from said annulus in responseto variations in inlet circulating pressure.

11. A system for controlling a well by maintaining constant bottomholepressure in the well through which drilling fluid is circulated fromapump to a drill string inlet and down the drill string and up theannulus between the drill string and the borehole wall to a surfaceannulus discharge comprising:

a closed flow path from said pump to said drill string inlet fordirecting all of the drilling fluid flowing from the pump into the drillstring inlet;

fluid pressure responsive means arranged downstream of said pump; and

throttle valve means arranged in said annulus discharge cooperating withsaid fluid pressure responsive means to vary automatically the rate offluid discharge from said annulus discharge in response to variations ininlet fluid pressure while maintaining input flow rate of drilling fluidconstant. a i

12. A system as recited in claim llv including means arranged in saidsystem for reducing the magnitude of the pressure variations resultingfrom variations in the rate of fluid discharge from said annulus inresponse to variations in inlet fluid pressure.

13;Apparatus for automatically maintaining constant bottomhole pressurein a well borehole during circulation of drilling fluid from a surfaceinlet down a drill string located in a borehole and up the annulusbetween the drill string and the borehole wall to a surface annulusdischarge which comprises:

a surface pump for pumping drilling fluid through said surface inlet,down said drill string and up said annulus between the drill string andthe borehole wall to said surface annulus discharge;

a closed flow path from said pump to said surface inlet directing all ofthe drilling fluid flowing from the pump into the surface inlet;

a pressure sensing means arranged downstream of said pump capable ofindicating changes in pressure of said drilling fluid;

an adjustable choke arranged in said annulus discharge line;

and

control means responsive to said pressure sensing means for controllinga said adjustable choke means so as to maintain an essentially constantbottomhole pressure in said borehole while maintaining input flow rateof drilling fluid substantially constant.

14. Apparatus as recited in claim 13 including means for interruptingtransmission of indications of said changes in drilling fluid pressureapproximately the amount of time required for a pressure pulse to travelfrom said adjustable choke down said annulus and up said drill string tosaid pressure sensing means.

15. Apparatus as recited in claim 14 in which said interruption means ispositioned between said pressure sensing means and said control means.

16. Apparatus as recited in claim'l4 in which said interruption means ispositioned in said control means.

17 Apparatus as recited in claim 14 in which said interruption means ispositioned between said control means and said adjustable choke.

18. A system for controlling a well by maintaining constant bottomholepressure in the well through which drilling fluid is circulated from asurface inlet down the drill string and up the annulus between the drillstring and the borehole wall to a surface annulus discharge comprising:

a pump for circulating said drilling fluid; fluid pressure responsivemeans arranged on said inlet;

throttle valve means arranged in said annulus discharge cooperating withsaid fluid pressure responsive means to vary automatically the rate offluid discharge from said annulus discharge in response to variations ininlet fluid pressure to maintain bottomhole pressure essentiallyconstant for a selected circulating pump rate; and means responsive tosaid pump output for automatically adjusting said throttle valve meansand thereby said drill string pressure an amount equivalent to changesin circulating friction resulting from changes in said selectedcirculating pump rate, thereby keeping bottomhole pressure essentiallyconstant for different fluid circulating rates.

19. A system as recited in claim 18 including means arranged in saidsystem for reducing the magnitude of the pressure variations resultingfrom variations in the rate of fluid discharge from said annulus inresponse to variations in inlet fluid pressure.

20. Apparatus for automatically maintaining constant bottomhole pressurein a well borehole during circulation of drilling fluid from a surfaceinlet down a drill string located in a borehole and up the annulusbetween the drill string and the borehole wall to a surface annulusdischarge which comprises:

a pressure sensing means capable of indicating changes in pressure ofdrilling fluid;

an adjustable choke arranged in said annulus discharge line;

control means responsive to said pressure sensing means for controllingsaid adjustable choke means so as to maintain an essentially constantbottomhole pressure in said well bore for a selected circulating pumprate; and

means responsive to said pump output and connected to said pressuresensing means for automatically adjusting said adjustable choke andthereby said drill string pres sure an amount equivalent to changes incirculating friction resulting from changes in said selected circulatingpump rate, thereby keeping bottomhole pressure essentially constant fordifferent fluid circulating rates.

21. A system as recited in claim 20 including means arranged in saidsystem for reducing the magnitude of the pressure variations resultingfrom variations in the rate of fluid discharge from said annulus inresponse to variations in inlet fluid pressure.

